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Leaving room for future expansion of panel

Is the rule of thumb to leave about 10% spares and approx. 30% spaces for future expansion in a panel? I don’t want to completely fill up panel. If so, I may have to go to a sub panel as close to all 42 slots in my three-phase panel may get filled up or close to that leaving little extra room for anything else. Right now I have a mix of one and three-phase loads in the panel. Is 42 slots the max for a panel or are there larger ones? Probably would then shift smaller loads such as lighting and receptacles to the sub panel to give me the flexibility for the future. Just want to make sure I leave enough room for expansion.

Is the rule of thumb to leave about 10% spares and approx. 30% spaces for future expansion in a panel? I don’t want to completely fill up panel. If so, I may have to go to a sub panel as close to all 42 slots in my three-phase panel may get filled up or close to that leaving little extra room for anything else. Right now I have a mix of one and three-phase loads in the panel. Is 42 slots the max for a panel or are there larger ones? Probably would then shift smaller loads such as lighting and receptacles to the sub panel to give me the flexibility for the future. Just want to make sure I leave enough room for expansion.

Most load center type panelboards probably only go up to 42 space, but you can now get them so they can take twins in every spot. I would try not to load up with twins though; its a lot of conductors to manage and some brands the twins are a lot more than the cost of 2 regular breakers (siemens regular Q line twins are the same price as 2 regulars though).

If you jump up to a true factory ordered panelboard, You can probably get just about whatever you want. You will pay a little more, not a ton though. I recently compared a 200 A MB load center with a panelboard, 30 something breakers. IIRC it was $700 for the loadcenter and $1000 for the panelboard.

Is the rule of thumb to leave about 10% spares and approx. 30% spaces for future expansion in a panel? I don’t want to completely fill up panel. If so, I may have to go to a sub panel as close to all 42 slots in my three-phase panel may get filled up or close to that leaving little extra room for anything else. Right now I have a mix of one and three-phase loads in the panel. Is 42 slots the max for a panel or are there larger ones? Probably would then shift smaller loads such as lighting and receptacles to the sub panel to give me the flexibility for the future. Just want to make sure I leave enough room for expansion.

There is no NEC requirement to leave room for expansion in a panel but IMO it is a good idea. You can get 60 circuit or 84 circuit, 3Ø panels.

20% is probably a good number for spare breaker capacity. If a panelboard is in a location where there are empty spaces that could be upfitted later you probably want to have more spare breaker capacity. As a rule we only use 30 space panels for 100 amp rated panelboards and 42 space for 200 plus amp rated panelboards. I've used double section panels with 84 spaces total. You can specify 48, 60 and other configurations too.